The present invention relates to a method of and an apparatus for correcting an edge interval of record signals in an optical recording/read-out apparatus in which digital signals are recorded on a recording medium such as an optical disk and the recorded signals are read out therefrom, wherein an edge interval is corrected for record signals written in the recording medium based on a power of the record signals and a record pattern written thereon immediately therebefore.
An optical disk has been employed is a recording medium on which digital or digitized signals are written. In the recording operation, a laser beam is focused onto a recording surface of the optical disk such that the intensity of the focused light is varied in relation to information to be stored thereon.
In this connection, there has been used a pit edge recording method in which the laser power is modified for a recording operation on the disk so as to store information before and after the record mark, and hence two or more data items are recorded for a record mark, which efficiently develops a high-density recording operation.
According to the pit edge recording method, in general, when a laser utilized in the recording operation has low power, the contour of the record mark thus created is likely to be unstable. In addition, when the recording speed is altered, the amount of thermal energy applied to a unitary area and the thermal distribution are varied, thereby disadvantageously generating various shapes of the record marks. In consequence, actually, in order to create a stable or uniform shape of the record mark for the recording and read-out operations, according to the "Application of Pit Edge Recording on PbTbSe Thin Film" found on page 4-176 of the Digest of the National Conference on 70th Anniversary of Foundation of the IECE of Japan, the operation is accomplished, for example, by adjusting the laser pulse length as follows. Namely, the recording laser is produced with an intensity slightly higher than the usual intensity such that the laser pulse length is shortened in the recording operation to suppress the excessive portion of the mark length associated with the line speed of the laser; moreover, in the read-out operation, the pulse length is reduced in a signal having undergone binarization.
Moreover, in general, the contour of the recorded mark primarily depends on the recording sensitivity and the thermal conductivity of the recording medium, the laser beam intensity distribution of the laser beam used for the recording operation, the wavefront aberration leading to a state where the actual focal state cannot be developed even when the laser beam is focused, and the like. Consequently, when a combination of the recording apparatus and the recording medium varies, the characteristics of the recording and read-out operations are also changed. In addition, in the recording operation, the level of the laser power of the recording apparatus changes as the time lapses. Even if an automatic power control (APC) mechanism is disposed to automatically control the laser power, there cannot be avoided a laser power level deviation in a certain range. This factor also varies the recording and read-out characteristics, which leads to a change in the record mark length in the recording operation and to a variation in a pulse interval of read-out signals in the read-out operation.
In consequence, for the recording apparatus of which the correction value of the laser pulse length and the laser power are respectively set to fixed values before delivery thereof to the users, the design specifications are decided depending on values of recording and read-out characteristics measured by use of many combinations of various recording media and recording apparatuses. In this operation, in order to guarantee a high reliability of the detection in many situations in consideration of a range of deviation of the recording and read-out characteristics due to the various combinations above, the recording density is determined with a large margin. Namely, the decision of the specifications is conducted with a sacrifice of the recording density.
In order to overcome this difficulty, there has been proposed a method in which to absorb the deviation in the characteristics due to the combinations of the recording media and the recording apparatus so as to develop a high recording density, a test pattern is beforehand recorded on an optical disk such that the test pattern is read out therefrom to attain information based thereon for adjustment of the recording conditions. For example, in an apparatus described in JP-A-61-239441, the laser power of the recording operation is adjusted and fixed to one level; in a device described in JP-A-61-74178, an adjustment amount of the recording pulse width is adjusted and fixed to one amount independent of the recording pulse width; and in an apparatus described in JP-A-61-304427, the fixed laser power of the recording operation, the fixed adjustment value related to and independent of the recording pulse width, and the automatic equalization coefficient of the read-out operation are simultaneously adjusted.
Furthermore, the recording operation of the optical disk is fundamentally accomplished through a thermal diffusion. Consequently, owing to preceding and following series of recording pulses associated with a record mark, the thermal distribution is diffused to cause a phenomenon in which the contour of the record mark is varied (to be called a thermal interference herebelow). This phenomenon also leads to a variation in the pulse interval of the read-out signals in the read-out operation. Consequently, in order to conduct an appropriate correction in the recording operation, the influence of the thermal interference is required to be taken into consideration. To cope therewith, according to the recording method described in JP-A-63-48617, the width of the record signal is changed depending on an edge interval of the preceding record signal. This method, however, is attended with the following problem.
Namely, when it is desired to increase the recording density up to a level where the record mark contour and the interval between record marks are of a size similar to the size of a laser spot focused onto a surface of the recording film, the range where the influence of the thermal interference of the optical disk is exerted exceeds the minimum length of the record mark used. In other words, a series of laser pulses adopted for the previous recording operation excercise influence on the shift of the edge position of a record mark to be next recorded on the disk. Particularly, in a case of a recording medium which has a high recording sensitivity with respect to an intensity of a laser beam and on which the recording operation can be hence accomplished with low laser power, a high thermal conductivity is developed in general and the thermal interference related thereto exerts influence thereon in a large range.
Moreover, in the adjustment method conducted in association with the edge interval of a record signal, since information related to the adjustment amount is preset in advance, the adjustment amount cannot be modified depending on a variation on the recording characteristic taking place in the course of the recording and the reading operation. In consequence, an adjusting error appears in relation to the discrepancy between the preset recording characteristic and the changed recording characteristic, which disadvantageously leads to an inappropriate adjustment.
On the other hand, in the method of attaining recording condition adjusting information as described in the three articles above, each of the recording laser power and the recording pulse width has an adjustment quantity of a single value. Consequently, it is impossible to minimize the variation of the recording mark length due to the thermal interference of which the magnitude varies depending on various record patterns.
Heretofore, since the signal read-out of a reproducing system has a narrow frequency bandwidth, a pulse signal reproduced has an expanding skirt or base portion. In this situation, in order to reduce linear interference occurring between neighboring signals due to a superimposed portion thereof, a linear equalizer such as a transversal filter is used on the reproduction side, in general, in the field of communications, magnetic recording, etc.
However, the influence of the thermal diffusion appears in the read-out operation primarily in the form of a shift of the waveform along the direction of time. This is associated with a nonlinear interference component of an interference between codes, which cannot be simply expressed by use of a linear superimposition of fundamental waveforms related to the record information. In consequence, the component of the edge position variation due to the thermal interference occurring in the recording operation cannot be coped with by a linear equalizer. Namely, actually, it is quite difficult for the system on the read-out side to appropriately deal with the interference component in a real time manner.
Particularly, in a pit edge recording method achieving an opto-magnetic recording operation on a recording medium having a high thermal conductivity, the mark length is associated with a large shift component and hence it is required to take a large margin for the component. This inevitably necessitates the sacrifice of the recording density i.e. the higher recording density cannot be implemented.